Based on the recent announcement of “A free, open-source future for Mailspring”, I would think that Mailspring could be listed as Open Source now in the Snap Store. If you go to snapcraft.io/mailspring you’ll see it listed as Proprietary. I realize that there has not been an update to the Snap since the announcement of open source but I think it would be a good idea to update the listing so people can be more easily aware of the current state.
As an aside, does Mailspring count as a open source product with a server that isnt open? At some point, you can disable the ID system making it locally only which would essentially make it only use open source pieces but hmm, I guess this thread is now a question about whether it can be changed on the Snap Store vs a suggestion to do so.